Adding a natural flavoring to formula?

Is there such a thing? My LO is getting really bored with his bottles lately. I've tried a sippy cup and he didn't even try to drink it out of that. What could I add to make it taste better for him? My mom told me syrup flavoring like chocolate or strawberry and I'm not down with that lol. I've looked on google and nothing really has caught my attention so I thought I'd ask y'all!

* I forgot to add this in. He is 10 almost 11 months old. He isn't teething at the moment. He eats regular food just fine but when it comes to a bottle.. nope!